One way or another, there tends to be duplicate Mark values in any given category, which produces warnings. Go to Manage --> Review Warnings to see the list for a given project. Some warnings are bad and should be corrected right away to maintain stability and optimal performance. Duplicate type marks are more of a nuisance. The warnings are good for door numbers, as we don't want duplicate door numbers on the same project, but for content that does not use the Mark value for anything, we tend to ignore the Mark value, which leads to a lot of unnecessary warnings.

We do get a warning right away when the problem first happens, but many users ignore those warnings unfortunately.

Type Mark Parameters

The Type Mark parameter does not auto increment, but if there are duplicates between families you will also get a warning as shown here; two different light fixture families with the same Type Mark value.

This warning is more serious and Revit puts this in your face, making you acknowledge it before moving on. Unlike the previous example where we can just ignore and keep working.

Family Editor Limitation

One of the most frustrating things about Mark and Type Mark is that they do not appear in the family editor (see image below). So, if you wanted to standardize to door/frame types or light fixtures it is not possible with the Type Mark parameter. You could have them all loaded into the template and set the Type Mark, but there are a lot of doors and light fixtures in the world, so this is not practical in my opinion (I know some do this, just my opinion).

To get around this limitation I recommend using Shared Parameters that replace the Mark and Type Mark parameters all together. This requires editing schedules and families, but once it is all setup, it works well. And, if you add the Shared Parameter as a Project Parameter and then associate it with all the categorizes it will appear in any manufacturer content you download. You just cannot preset those values, of course.

There is more to be said about this, but I wrote this all this morning and have to be done now!